"Unexpected drawing version: 39" (or similar) error or "Object reference not set to an instance of an object" error when opening the software.
This error can occur when you are trying to open a model created in a more recent version of the software than the version that you have, or when one of the supporting files (such as output or DWH drawing file) is corrupt.
First, check if the version that you're using is greater than or equal to the version that the model was created in (look at See Also section below). You will need to upgrade to the version of the software used for this issue or upgrade to the latest version of the software if you are using an older version.
If you're using a version of the software that's greater than or equal to the version that the model was last saved in, check the support files. In the same folder as the model (or in the results file path specified under Tools > Options > Project), you may see several files beside the main project files. To ensure that they are not the cause of the problem, copy the main two project files (either .wtg and .wtg.sqlite or .wtg and .wtg.mdb, depending on your version) to a different folder and try opening it from there. We have seen cases where the .DWH file can sometimes be the issue and it can safely be deleted because it is regenerated when reopening the model.
You should make sure that you have the latest patch set applied to the software. You can find directions on how to do that here.
If the above does not work you can also try to import that database file into a new model, by going to File > Import > WaterCAD database.
Find the version a model was created in
Errors when opening model saved in a newer version
What files are needed to open a model in an OpenFlows product?